Want to show your readers what book you're reading? That's just what this plugin does coincidentally.
The function current_book is defined within the bookdisplay.php file and can be manipulated there with a basic understanding of the PHP programming language.
Do to its simplicity the plugin should work for almost (if not) all versions of WordPress. It has been thoroughly tested in the current WordPress release (2.5). If you find a bug in any other versions of WordPress, please let me know so that I may fix them.